1. Field of the Invention
The present invention relates to a multiplex transmission system used in, e.g., a control system of a vehicle.
2. Summary of the Invention
A conventional multiplex transmission system for a vehicle, which uses a transmission system called CSMA/CD (Carrier Sense Multiple Access/Collision Detection), is constituted by connecting a plurality of multiplex transmission nodes to each other via a multiplex transmission path. Upon transmission/reception of data, each communication node writes necessary data in a predetermined data area in a predetermined communication frame, and transmits the communication frame via the multiplex transmission path, thus exchanging data representing operation information, and the like of a vehicle.
For example, Japanese Patent Laid-Open No. 1-143533 discloses, as a communication frame format for a multiplex transmission node, a communication frame format constituted by an SD (Start Delimiter) area, a destination address area, a source address area, a frame ID area, a data length write area, a data write area, and a check code write area, as shown in FIG. 8. The SD area in this reference is an area in which a specific code (SD code) indicating the start of a communication frame is written, the destination address area is an area in which the address of a communication node which is to receive this communication frame is written, the source address area is an area in which the address of a communication node at the transmitting side of this communication frame is written, the frame ID area is an area in which data for specifying the contents of communication data to be transmitted by this communication frame is written, the data length write area is an area in which the number of data written in this frame is written, the data write area is an area in which specific data to be transmitted by this frame is written, and the check code write area is an area in which a check code indicating the end of this frame is written.
Since the communication frame with this format includes the destination address area, this communication frame can be prevented from being received by a communication node other than a communication node designated by the destination address area. Since this destination address area is assigned, when a data sendback request is to be transmitted to a specific communication node using this communication frame, transmission of an unnecessary signal from another communication node can be prevented.
However, in the above-mentioned conventional multiplex transmission system, even when the system performs normal communications as a whole and the destination address need not be especially specified, since the frame includes the destination address area, the frame length is large, and for this reason, the busy time of a multiplex transmission line is prolonged. A communication node which received the frame including the destination address area must read the address written in the destination address area in the frame each time it receives a frame, and must check if this communication frame is addressed to its own node, thus prolonging the discrimination time.
For this reason, a multiplex transmission system which adopts a system called a BROADCAST system using a communication frame from which a destination address area is omitted has been proposed.
However, in this BROADCAST multiplex transmission system, in particular, when a communication node is initialized (reset), data stored in the communication node is cleared. For this reason, when the reset node wants to request necessary data from another communication node, since the amount of data to be requested becomes large, the transmitting side must designate a node to which a transmission request is to be issued and data to be transmitted from the designated node. However, in the conventional BROADCAST system, since the frame has no destination address area, a node to which a transmission request is to be issued cannot be designated.